Measurements of Freestream Fluctuations in the NASA Langley 20-Inch Mach 6 Tunnel

Abstract

An experimental campaign was conducted to measure and to characterize the freestream disturbance levels in the NASA Langley Research Center 20-Inch Mach 6 Wind Tunnel. A pitot rake was instrumented with fast pressure transducers, hot wires, and an atomic layer thermopile to quantify the fluctuation levels of pressure, mass flux, and heat flux, respectively. In conjunction with these probe-based measurements, focused laser differential interferometry was used to optically measure density fluctuations. Measurements were made at five nominal different unit Reynolds numbers ranging from 3.28-26.5×10 6 /m. The rake was positioned at two different streamwise locations and several different roll angles to measure flow uniformity within the test section. In general, noise levels were spatially consistent within the tested region. Pitot pressure fluctuation levels ranged from 0.84% at the highest Reynolds number tested to 1.89% at the lowest Reynolds number tested. Freestream mass-flux fluctuations remained relatively constant between 1.8-2.5% of the freestream. The pressure transducers were also used to determine the dominant disturbance speed and angle of propagation. The disturbances were estimated to travel at approximately 54-81% of the freestream speed at an angle of approximately 21-44°from the freestream direction, but these measurements had a significant amount of uncertainty. A comparison to previous measurements of pressure made in 2012 and of mass flux made in 1994 show almost no change in the RMS fluctuation of these flow quantities.

Simultaneous focused laser differential interferometry and high-speed schlieren in a Mach 6 flow

“…The work reported in [16] was first performed with a conventional, single-point FLDI instrument shown in figure 1(a), where the continuous wave laser beam propagated along the optical axis (z-axis), oriented perpendicular to the wind tunnel's freestream flow (x-axis). Details pertaining to the 20-Inch Mach 6 Tunnel facility are found in [17].…”

“…After performing measurements reported in [16] with the conventional single-point FLDI instrument upstream of physical pressure probes without schlieren, a colinear HSS capability was then added to the FLDI setup, shown schematically in figure 1(b). Note that for this configuration, the FLDI laser propagated from the transmitter to receiver side of the instrument while light from the HSS was counterpropagating.…”

“…Additionally, facility operators may require schlieren visualization as a means of monitoring flow quality, which may otherwise be obscured by the presence of another instrument. In this work, a modified version of the FLDI instrument in [16] is used to demonstrate simultaneous FLDI/HSS density gradient measurements over the same line-of-sight. Simultaneous measurement of freestream density fluctuations with FLDI and pitot probe shock standoff distance with HSS were acquired near the end of a recent wind tunnel calibration test [16] in NASA Langley Research Center's 20-Inch Mach 6 Tunnel.…”

“…In this work, a modified version of the FLDI instrument in [16] is used to demonstrate simultaneous FLDI/HSS density gradient measurements over the same line-of-sight. Simultaneous measurement of freestream density fluctuations with FLDI and pitot probe shock standoff distance with HSS were acquired near the end of a recent wind tunnel calibration test [16] in NASA Langley Research Center's 20-Inch Mach 6 Tunnel. A series of laboratory tests were then performed to verify that the inclusion of the HSS system optical components did not interfere with the FLDI system measurements.…”

Measurements of freestream density fluctuations in a hypersonic wind tunnel

“…FLDI has a streamwise spatial resolution of the order of hundreds of microns, and tens of millimetres in the spanwise direction. FLDI has been used for facility characterisation via freestream density fluctuation measurements in a reflected shock tunnel (Parziale et al, 2014) and in blowdown facilities (Fulghum, 2014;Chou et al, 2018). FLDI has also been used to investigate boundary layer instabilities in hypervelocity flows (Jewell et al, 2016) owing to its high bandwidth and high streamwise spatial resolution.…”